The Hydrolysis of Zn-ions - ACS Publications

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The Hydrolysis of Zn-ions: Controllable Synthesis of ZnCo (OH)F Nanostructures with its Electrochemical and Optical Properties Shuo Wang, Baoqiang Zhang, Chunlan Gao, Jingdong Guo, and De'an Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00280 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

The Hydrolysis of Zn-ions: Controllable Synthesis of ZnxCo1-x(OH)F Nanostructures with its Electrochemical and Optical Properties Shuo Wang, Baoqiang Zhang, Chunlan Gao, Jingdong Guo and De’an Yang*

AUTHOR ADDRESS Key Laboratory for Advanced Ceramics and Machining Technology Ministry of Education, School of Materials Science and Engineering, Tianjin University, Tianjin, 300350, P.R. China.

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ABSTRACT: Recently, the synthesis and property of metal hydroxide fluorides have attracted more and more attentions. In this paper, a series of bimetallic hydroxyl fluorides, ZnxCo1-x(OH)F, were grown on functionalized carbon fiber paper via a template-/binderfree hydrothermal method, whose nanostructures can be controlled only by adjusting the adding of Zn2+. Based a series of experiments, the mechanism of morphology-evolution is proposed firstly, in which the Zn2+ hydrolyze under hydrothermal conditions to form colloidal micelles, flocculate and adsorb with other ions. The mechanism of Znhydrolysis has been verified in other substrates, providing a novel thinking of the design of nanostructure. What’s more, the electrochemical properties of the obtained ZnxCo1x(OH)F

have been studied, illustrating it processes redox reactions and ion

adsorption/desorption simultaneously as a quasi-pseudocapacitance materials. On the other hand, the optical properties have been analysed by UV-visible absorption spectrum, in which the absorption peaks can be observed around 494 nm, 526 nm and 628 nm in visible-light region. The above properties illustrate that the obtained ZnxCo1-x(OH)F have potentials in various fields as a novel material, such as electrochemical, physical or optical fields.


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1

Introduction Recently, the synthesis and characterization of metal hydroxyl halides have attracted

much attention.1-2 The thermal stabilities of the hydroxyl halides have been reported in early stage3, illustrating the halide ion have effort on their properties. Secco et al.4-5 synthesized a series of metal hydroxyl halides and compared their physical or chemical properties. Among the halides, fluoride ions possess almost similar ionic radius as hydroxide ion, resulting in small lattice deformation. Besides, the amphoteric behaviour of hydroxyl ion can be balanced by the fluoride ion because of its high electronegativity.6 Therefore, the metal hydroxide fluorides have attracted more attention compared with other hydroxyl halides. Ling et al.7 synthesized and determined the crystal and magnetic structures of Fe(OH)F and Co(OH)F, illustrating these crystallize with distorted ramsdellite-type structure similar to α-AlOOH, in which the Fe2+ or Co2+ are located in octahedral symmetry (M(O,H)6). Zn(OH)F have been utilized as a catalyst for the formation of pyridine or a precursor for the synthesis of ZnO8, in which the intrinsic properties have been ignored. In recent years, the synthesis and properties of Zn(OH)F nanostructures have been reported. You et al.9 synthesized the nanowire network of Zn(OH)F and studied its luminescent and photocatalytic properties. Cao et al.10 synthesized flower-like Zn(OH)F via a microwave method with the assistant of ionic liquids. Compared with Zn(OH)F, the synthesis and electrochemical properties of Co(OH)F have attracted more attention recently because of the high theoretical capacitance and excellent electrochemical activity of cobalt-based materials.11 Cao et al.12 designed the hierarchical superstructure of Co(OH)F, which can be used as an electrocatalyst for water



splitting. Ding et al.13 synthesized the hybrid hierarchical Co(OH)F/Ni(OH)2 as electrodes for supercapacitors. Single metal hydroxyl fluorides, such as Co(OH)F and Zn(OH)F, have been reported extensively but the synthesis of bimetallic hydroxyl fluorides was rarely reported. Dong et al.14 synthesized Ni0.33Co0.67(OH)F hollow hexagons woven by MWCNT for lithium-ion batteries, which is the only report on bimetallic hydroxyl fluorides. It is generally known that slow ion diffusion rate and limited specific surface area of the bulk counterpart restrict the improvement of the properties.12 The design of various nanostructures can be utilized to modify these problems, which enhances the active surface area and improves the ion diffusion rate. During the design of various nanostructures, the effects of surfactants and precipitants are focused in almost all literatures15-16 while the intrinsic properties of the metal ions are generally ignored. On the other hand, the directly growth of active materials on conductive substrates has attracted much attention. The interfacial contact between active materials and current collectors can be improved using this binder-free method.17 Transition metal oxides/hydroxides have been grown on various conductive substrates and possess excellent electrochemical properties. Inspired by the above considerations, we synthesize ZnxCo1-x(OH)F (0≤x≤0.67) on functionalized carbon fiber papers (CFP), using a facile hydrothermal method. Only by adjusting the adding of Zn2+, a series of nanostructures can be controlled on the substrates, such as urchins, flowers, and sisals. The hydrolysis of Zn2+ plays significant role in the evolution of these morphologies, which is firstly proposed in the design of nanostructures. The proposed mechanism has been verified further utilizing other substrates such as


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nickel foams and titanium meshes. The obtained composites possess the following advantages. On the one hand, the ion diffusion rate and electron conductivity can be improved by the nanostructures directly grown on substrates. On the other hand, the presence of F- in ZnxCo1-x(OH)F improves the conductivity and increases the charge mobility meanwhile the OH- contributes the wetting of electrolytes.1 What's more, the optical properties of the obtained ZnxCo1-x(OH)F have been studied, in which the absorption peaks can be observed around 494nm, 526nm and 628nm. In an octahedral crystal field, the 4F ground state for Co2+ is split into 4T1g(P) and 4A2g levels and can be assigned to the above absorption peaks. Besides, these absorption peaks decrease gradually with the increasing of Zn2+ content, proving the adding of Zn2+ simultaneously. To the best of our knowledge, this is the first report on the synthesis of ZnxCo1-x(OH)F and the role of Zn-hydrolysis in aqueous solutions, which providing a novel thinking of the design of various nanostructure. Besides, the optical or spectral properties of metal hydroxyl fluorides, especially the bimetallic hydroxyl fluorides, have been not reported before. 2

Experimental Section 2.1

Materials

All chemicals were of analytic grade mentioned otherwise. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), urea (CO(NH2)2), ammonium fluoride (NH4F), concentrated sulfuric acid (H2SO4), concentrated nitric acid (HNO3), concentrated hydrochloric acid (HCl), anhydrous ethanol (CH3CH2OH) and acetone (CH3COCH3) were purchase from Tianjin Yuanli Chemical Co., Ltd (China) without further purification. Carbon fiber paper (CFP, TGP-H-090) were purchased from



Toyolac Inc., (Japan) and functionalized as described below. Nickel foam (NF, 110 ppi) and titanium mesh (TM, 100 mesh) were purchased from Shanxi Lizhiyuan Battery Material Co., Ltd (China) and Hebei Kangwei Metal Co., Ltd (China) respectively. 2.2

Functionalization of carbon fiber paper

A piece of CFP (2×1 cm2) was firstly sonicated in acetone to remove the impurities. After that, the treated CFP was rinsed with deionized (DI) water to remove the residual acetone. Then, the cleaned CFP was immersed in mixed concentrated acid solution of H2SO4: HNO3 (3:1 v/v) and the solution was heated at 60 ℃ for 1 h. The functionalized CFP was washed with DI water to remove the residual acid and sonicated in DI water and ethanol successively. The obtained functionalized CFP was dried in vacuum oven at 60 ℃ for 8 h finally. 2.3

Pretreatment of nickel foam and titanium mesh

A piece of nickel foam (NF) or titanium mesh (TM) (2×1 cm2) was sonicated in acetone, different concentration HCl, DI water and ethanal successively, in which the concentration of HCl for NF is 6 M while TM is 12 M. The obtained clean NF or TM was dried in vacuum oven at 60 ℃ for 8 h finally. 2.4

Synthesis of ZnxCo1-x(OH)F on substrate

The mole ratio of M2+(NO3)2·6H2O: CO(NH2)2: NH4F was kept as 3:6:5, the total of Co(NO3)2·6H2O and Zn(NO3)2·6H2O was 3 mmol for all samples only changing the adding of Zn. In a typical synthesis, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, CO(NH2)2 and NH4F were dissolved into 80 ml DI water with magnetic stirring to form a clear pink solution. Subsequently, the functionalized or pretreated substrate was stuck to a 100 ml Teflon-lined autoclave. The mixed solution was transferred to the above autoclave and


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heated in an electric oven at 120 ℃ for 6 h. After cooling down to room temperature, the composited substrates were washed with DI water and dried in vacuum oven at 60 ℃ for 10 h. In order to obtain different nanostructures, the contents of Zn2+ were controlled as x=0, 0.2, 0.25, 0.33, 0.5, 0.67, 0.75, 0.8 and 1. For the supporting experiments, the experiments were also conducted under similar conditions only changing the types of the reactants. As for the heat-treatment products, the as-synthesized samples were put in a porcelain boat and heated for 200 ℃, 300 ℃, 400 ℃ and 500 ℃ in air for 2 h with a heating rate of 5 ℃/min. 2.5

Characterization

Scanning electron microscopy (SEM, Hitachi S4800, Japan) was used to characterize the morphologies of the products, which were cut into small pieces and pasted on sample stage by conductive adhesive. The morphology of the samples was further investigated by transmission electron microscopy (TEM, JEM 2100F, Japan). Before the examination, the ZnxCo1-x(OH)F were diluted with ethanol, sonicated, and dropped onto the carboncoated copper grids. The crystal structure of products was studied by X-ray diffraction (XRD, Bruker D8 Advanced and Rigaku MiniFlex600) at room temperature (Cu Kα radiation, λ=1.5418 Å). The valence states of various elements were analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer Corp. PHI-1600, USA) at room temperature (Al Kα radiation, hν=1486.6 eV, calibrated by C 1s). In order to avoid the influence of the CFP, the obtained ZnxCo1-x(OH)F were scraped from the substrates for the measurements. UV-visible absorption spectrum (UV-vis, Shimadzu UV-1800, Japan) was used to record the optical absorption spectra and calculate the band energy. In order



to avoid the block of CFP, the powers were scraped from the substrates, and then diluted in ethanol. 2.6

Electrochemical tests

The electrochemical measurements were carried out at room temperature in a standard three-electrode with a 1.0 M KOH aqueous solution as the electrolyte. The obtained ZnxCo1-x(OH)F on CFP were used directly as the working electrode, a platinum plate as counter electrode and saturated calomel electrode (SCE) as the reference electrode. All electrochemical measurements such as cyclic voltammetry (CV) and galvanostatic charge/discharge measurements (GCD) were performed utilizing a CHI660E (CH Instruments, China) electrochemical workstation at room temperature. Before the testing, the obtained electrodes are activated by cyclic voltammetry in same electrolyte. The cyclic voltammograms (CV) were recorded in the potential range between 0 and 0.5 V vs. SCE at the various scan rates of 2, 5, 10, 20, 30, 50 and 80 mV/s and the galvanostatic charge/discharge tests (GCD) were measured in the potential range between 0 and 0.4 V vs. SCE. 3

Results and discussion 3.1

The synthesis of ZnxCo1-x(OH)F on substrates

As shown in Fig. 1a, all the samples have similar XRD patterns indexed to Co(OH)F12 (JCPDS No.50-0827) and the details are provided in supporting. Besides, the main peak (20.835°, (110)) have been magnified in Fig. 1b, in which this peak shift to lower angles gradually when the increase of Zn2+ because of the difference of the ionic radius.18 The values of the lattice parameter a have been found to reduce linearly in Fig. 1c with the increase of Zn2+.19-21 On the other hand, a facile heat-treatment process is used to identify


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the adding of Zn2+ further, which is provided and discussed in supporting. Because of the identical XRD pattern and similar crystal structure, Zn0.33Co0.67(OH)F (x=0.33) was utilized to XPS and the Co 2p, Zn 2p and F 1s spectrums of the powders are shown in Fig. 1d-f. The Zn 2p core level spectrum can be divided into two peaks, the strong peaks at 1021.4 eV and 1044.8 eV corresponding to Zn 2p1/2 and Zn 2p3/2 of Zn2+. The Co 2p core level spectrum shows four peaks: The binding energy at 781.0 eV with its satellite peak at 786.2 eV correspond to Co 2p1/2, whereas the binding energy at 796.9 eV and its satellite peak at 802.5 eV correspond to Co 2p3/2 of Co2+. Besides, the sole strong peak of F 1s at 683.6 eV can be utilized to demonstrate the (OH)-F.22-23 The results of XRD and XPS illustrate the synthesis of ZnxCo1-x(OH)F.

Figure 1. (a) and (b) XRD patterns of the obtained ZnxCo1-x(OH)F (0≤x≤0.67), (c) lattice parameter a of ZnxCo1-x(OH)F (0≤x≤0.67) with the adding of Zn2+. XPS spectrums of Zn0.33Co0.67(OH)F (x=0.33): (d) F 1s, (e) Zn 2p, (f) Co 2p. To explore the influence of the content of Zn on the morphology of ZnxCo1-x(OH)F (0≤x≤0.67), a series of content-dependent experiments have been carried out and the



evolution of nanostructures with the increasing of Zn can be seen clearly in Fig. 2 when utilizing CFP as substrates. Urchins-like made up of radial nanowires can be seen in Co(OH)F (x=0). After adding Zn, the morphologies of ZnxCo1-x(OH)F (x=0.2, 0.25, 0.33) gradually change, in which the tips of the nanowires become adjacent and out-of-order. When the content of Zn equals to Co (x=0.5), the morphology of sisals consisting of nanowires and nanosheets is seen except urchins. Flowers-like structures can be formed adding Zn to ZnxCo1-x(OH)F (x>0.5) further, but the microrods have been increasing gradually when adding Zn excessively (x≥0.67) in Fig. S2.

Figure 2. SEM images of ZnxCo1-x(OH)F on CFP with the adding of Zn: (a)x=0; (b) x=0.2; (c) x=0.25; (d) x=0.33; (e) x=0.5; (f) x=0.67. TEM was used to identify the microstructure of ZnxCo1-x(OH)F, and Zn0.33Co0.67(OH)F (x=0.33) was utilized to test because of the similar crystal structure and morphology. The HRTEM of nanowires in Fig. 3c revealed interplanar spacings were 0.25 and 0.28 nm, corresponding to (111) and (310) lattice planes of Co(OH)F, respectively. The angle of (111) and (310) lattice planes can be measured as 59°, in according with theoretical value


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of the corresponding lattice planes of orthorhombic Co(OH)F. Because of the existence of F and Zn, the crystal lattice can be easily changed under the transmission electron beams, making it difficult to get the HRTEM and SAED pattern9.

Figure 3. (a), (b) TEM and (c) HRTEM of Zn0.33Co0.67(OH)F (x=0.33). 3.2

The mechanism of morphology-evolution

From the SEM with the adding of Zn, the content of Zn2+ plays significant role in the morphology-evolution. In order to study this morphology-evolution mechanism, a series of supporting experiments have been carried and the results are discussed as follows. Zn2+ and Co2+ react with CO(NH2)2 and NH4F respectively to study the different role of reactants in hydrothermal process. The morphologies using different reactants are shown in Fig. S3-5. Firstly, CO(NH2)2 was chosen as precipitant solely. There were continuous networks consisting of nanosheets adding Zn2+ into CO(NH2)2. When Co2+ reacting with CO(NH2)2, uniform nanosheets can be synthesized on the substrates. Then, NH4F was utilized as precipitant. The rhombus columns can be seen lonely only taking Zn2+ as metal ions. When Co2+ reacting with NH4F, there were a lot of prisms with considerably small sizes. Besides, the synergetic effect of CO(NH2)2 and NH4F were studied. Urchins-like can be found using Co2+ solely while flowers-like can be seen adding Zn2+ to the precipitants. Based on the SEM, CO(NH2)2 and NH4F play various role in hydrothermal process: the former prefers to generate nanosheet structure while the latter contributes to nanowire/column structure. Moreover, the size of the products using Zn2+ as metal cations is bigger than Co2+ when using same precipitants. Especially, the morphologies are



obviously distinct using Zn2+ or Co2+ as metal cations, which might be caused by the hydrolysis of Zn-ions. On the other hand, XRD were used to identify the crystal structure of the products utilizing various reactants so the results are shown in Fig. S6. When using CO(NH2)2 as precipitants lonely, the products can be indexed to zinc or cobalt carbonate hydroxide but it is hard to match with PDF because of the missing of databases. Replacing CO(NH2)2 with NH4F, the products can be indexed to zinc or cobalt hydroxide fluoride accurately. Adding CO(NH2)2 and NH4F as precipitants simultaneously, the products can be indexed to zinc or cobalt hydroxide fluoride with weak crystallinity. The XRD results are in according with the SEM images: The zinc or cobalt hydroxide fluorides using NH4F as precipitants with micrometer size have grown adequately to high crystallinity while the zinc or cobalt hydroxide fluorides adding CO(NH2)2 and NH4F with nanometer size grow inadequately to weak crystallinity. Based the above results, the hydrolysis of Zn2+ in aqueous solution plays significant role in the evolution of various morphologie. Based on this supposition, the mechanism of morphologyevolution is proposed in Scheme. 1 as follows. With the increase of temperature, CO(NH2)2 and NH4F could generate more OH- and F- at the same time Zn2+ hydrolyze severely. Without Zn2+ ions, the OH- and F- react with Co2+, nucleate on the substrates and grow along one direction to form nanowires, in which the urchins are formed on the substrates directly. When adding Zn2+ in aqueous solution, Zn2+ hydrolyze to form colloidal micelles and adsorb with each other easily, in which the vortexes are generated by uneven ion concentration and the out-of-order flowing. The unstable Zn-micelles with high surface energy prefer to nucleate on the substrates, flocculate and adsorb with other ions, resulting the nanowires adjoin with others even convert into nanosheets. While adding Zn2+ into solutions excessively, the Zn-micelles flocculate, nucleate and grow 12


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quickly, generating rhombus columns with high crystallinity. The Zn-micelles increase quickly with the increasing of Zn2+, in which it is easy to nucleate and precipitate directly in solution other than the substrates.

Scheme 1. The fabrication and morphology-evolution under the influence of the hydrolysis of Zn2+ without the substrates. The synthesized ZnxCo1-x(OH)F can be grown on nickel foams and titanium meshes in Fig. S78 using the above method, confirming the proposed mechanism of Zn-hydrolysis further. Compared with CFP, the rough surfaces of the NF and TM have significant influence on the absorption and nucleation of the ions. On the other hand, the strong interaction forces between the substrates and the nanostructure cause it difficult to grow only by the crystallinity habits or Ostwald ripen. What’s more, the active metal-base substrates like NF may be corroded in hydrothermal process or reacted with the reactants in solutions, which make it difficult to analyze the reaction mechanism. 3.3

The electrochemical properties of ZnxCo1-x(OH)F on CFP

The electrochemical properties of the obtained ZnxCo1-x(OH)F/CFP were evaluated in a 1.0 13



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M KOH aqueous solution using a three-electrode system at room temperature. The CV plots with scan rate of 2 mV/s can be found in Fig. 4a, in which two pairs of redox peaks can be seen and assigned to the redox reactions of Co3+/Co2+ and Co3+/Co4+.24 The symmetrical images of the GCD plots in Fig. 4b illustrate the reversible behavior of the ZnxCo1-x(OH)F/CFP electrode and the slope changes in changing/discharging process correspond to the redox peaks in CV plots. The details of the ZnxCo1-x(OH)F/CFP can be found in Fig. S9 and the obtained ZnxCo1x(OH)F/CFP

without obvious redox peaks and charging/discharging platforms can be seen as

quasi-pseudocapacitance materials, which process redox reactions and ion adsorption/desorption simultaneously. However, the IR drops can been seen clearly in in changing/discharging process because of the intrinsic low conductivity of the ZnxCo1-x(OH)F, resulting the decline of the specific capacitances with the increase of current densities in Fig. 4c. Fig. 4d shows the cycling performance of the ZnxCo1-x(OH)F/CFP at 1 A/g in three-electrode system. As seen in the figure, the specfic capacitances remain stable greatly after the initial degradation. From the above results, the electrochemical properties of the ZnxCo1-x(OH)F on CFP have been influenced by the synergistic effect of the compositions and the hierarchical nanostructures, especially the intrinsic properties. But the specific capacitances of the ZnxCo1-x(OH)F have been restricted by the intrinsic conductivity, so the next steps are concentrated on improving the intrinsic conductivity or compositing it with high conductive materials.

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Figure 4. (a) CV curves with a scan rate of 2 mV/s, (b) GCD plots at a current density of 0.5 A/g, (c) specific capacitances at different current densities, and (d) cycling performances of the ZnxCo1-x(OH)F on CFP (0≤x≤0.67) at 1 A/g. 3.4

The optical properties of ZnxCo1-x(OH)F

The optical properties of the obtained ZnxCo1-x(OH)F were studied by UV-Vis absorption spectrum in Fig. 5a, in which the absorption edge of the Zn(OH)F is seen to the lower energies in ZnxCo1-x(OH)F (0≤x

The Hydrolysis of Zn-ions - ACS Publications

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